The present invention relates to a process of preparing fluorinated material preferably employed as raw material of a gas diffusion electrode of a fuel cell.
The gas diffusion electrode mainly employed in a fuel cell is an indispensable factor of effectively conducting a gas-liquid phase reaction. In the gas diffusion electrode in which fine channels of two types for a gas and an electrolyte are present, it is extremely difficult to achieve a long life and high performances in the existing structures of the fuel cell in which are used the conventional polytetrafluoroethylene (hereinafter referred to as "PTFE") for wet-proofing of gas channels because the gas channels are easily filled with an electrolyte in a short period due to the low wet-proofing property and become unable to supply a reactant gas to catalyst clusters existing in the electrolyte channels. By increasing the PTFE content, the hydrophobic property can be improved, but the utilization of catalyst clusters for the electrode reaction simultaneously decreases, resulting in the reduction of the electrode performance. Namely, the requirements of the gas and electrolyte channels to improve the respective properties contradict each other with the use of the conventional materials because their insufficient hydrophobic property.
In order to solve the above problem, one of the present inventors has proposed a process of preparing fluorinated particulate material employed in a fuel cell (U.S. Pat. No. 5,137,754). The process comprises applying a hydrocarbon polymer solution (for example, a hexane solution) to the surface of support material, vaporizing the solvent and fluorinating the polymer compound. The fluorinated material provided by this process, when employed in the fuel cell, forms a network through which a gas efficiently permeates.
In the network provided with material other than the above which is composed of carbon black of which a particle size is usually about 0.02 and 0.06 .mu.m and PTFE, particles of which a particle size is usually about 0.3 .mu.m, the carbon black surface is not completely covered with PTFE. The space in the agglomerated carbon black or that between the carbon black and PTFE may act as a gas network at the initial stage of operation, but the space will be filled with an electrolyte sooner or later resulting in the degradation of cell performances.
Since, to the contrary, a thin layer is formed around the material provided by the process of the U.S. patent, the fuel cell employing the material exhibits high performances because the space is not likely to be filled with the electrolyte.
However, the material particles before the fluorination of the U.S. patent are likely to agglomerate with each other to become massive so as to provide the thin layer of which a thickness is not uniform so that time complete fluorination can be achieved only under strict conditions. Since, further, the material becomes massive, the space or fine apertures of the materials is likely to be filled with the hydrocarbon polymer to lessen the number of channels for gas supply and discharge. In addition, the preparation of the solution of the hydrocarbon polymer itself is dangerous and rather complex.